Abstract: We present here our observations and analysis of the dayside emission
spectrum of the hot Jupiter WASP-103b. We observed WASP-103b during secondary
eclipse using two visits of the Hubble Space Telescope with the G141 grism on
Wide Field Camera 3 in spatial scan mode. We generated secondary eclipse light
curves of the planet in both blended white-light and spectrally binned
wavechannels from 1.1-1.7 micron and corrected the light curves for flux
contamination from a nearby companion star. We modeled the detector systematics
and secondary eclipse spectrum using Gaussian process regression and found that
the near-IR emission spectrum of WASP-103b is featureless across the observed
near-IR region to down to a sensitivity of 175 ppm, and shows a shallow slope
towards the red. The atmosphere has a single brightness temperature of T_B =
2890 K across this wavelength range. This region of the spectrum is
indistinguishable from isothermal, but may not manifest from a physically
isothermal system, i.e. pseudo-isothermal. A Solar-metallicity profile with a
thermal inversion layer at 10^-2 bar fits WASP-103b's spectrum with high
confidence, as do an isothermal profile with Solar metallicity and a
monotonically decreasing atmosphere with C/O>1. The data rule out a
monotonically decreasing atmospheric profile with Solar composition, and we
rule out a low-metallicity decreasing profile as non-physical for this system.
The pseudo-isothermal profile could be explained by a thermal inversion layer
just above the layer probed by our observations, or by clouds or haze in the
upper atmosphere. Transmission spectra at optical wavelengths would allow us to
better differentiate between potential atmospheric models.